I am feeding my 5 month old lab mix puppy Blue Buffalo Wilderness puppy food. Since the last 3 weeks he has become very picky and only eat a tiny bit of doesn't eat his food at all.
Last week the vet said he was a bit under weight and to switch him to the Blue Buffalo puppy chow for large breeds. He still is not eating much, maybe 1 cup twice a day. And a bit more every 4-5 days.
He is drinking water and seems to have his typical energy level. He is teething and the weather was very very hot last week.
He eats treats and will run to lick the cat bowls if they have been left with any tiny morsel.
Should I be worried? He's not skinny, but I wish he was a bit fuller.

The heat may have affected his appetite.

How much exercise are you giving him? Wilderness is a very high protein diet. If he is not using those calories he is just storing them making him not very hungry.

He may not like the texture of large breed puppy formula. You went from a meat based diet to run of the mill type of dog food.

Stop giving him treats and be extra careful he isn't getting the cat food. If everything else about him is fine, than let him get hungry. Start a schedule. Put the food down in the morning for 15-20 minutes.. if he is picking, pick it up and he gets NOTHING for the rest of the day until dinner, than do the same thing. Put the food down for 15-20 minutes.. if he is picking at it or walks away, put the food up.

If he goes 3-4 days of not eating or picking, than its time to switch puppy foods. Sometimes the holistic/organic puppy foods just aren't very tasty. I haven't met many puppies that like them. You can still find him a good food that he will eat. I've always had luck with Purina Pro Plan, Nutro puppy and Iams. Those are not ideal puppy foods, but they are nutritionally balanced and not ridiculously expensive... and for the most part, puppies like the taste. When puppies turn a year, than start feeding the holistic/organic foods.